1. Field of the Invention
The present invention relates to a multiple signal transmission method and a multiplexer/demultiplexer for multiplex-transmission of data processed by a computer or the like. More specifically, the invention relates to a multiple signal transmission method and a multiplexer/demultiplexer for multiplexing data coded in an 8B/10B block in a SONET/SDH frame, and then demultiplexing the data.
2. Description of the Related Art
Through SONET/SDH Network, a number of signals can be efficiently multiplexed to be transmitted for a long distance at a high speed in accordance with internationally standardized multiplexing steps. Conventionally, most of traffic transmitted through a network has been voice traffic. Thus, voice signals of mainly 64 Kbps have been multiplex-transmitted through the SONET/SDH Network.
However, because of a recent increasing ratio of data traffic caused by popularization of Internet, the network is now required to transfer data traffic. The SONET/SDH Network is also required to multiplex-transmit data signals.
It is especially desired that the SONET/SDH Network transmit LAN traffic between remote points in transparency, storage data for a long distance, and digital video signals. Examples are Gigabit Ethernet used for LAN, Fibre Channel and Enterprise System Connect (ESCON) used for a storage area network, Digital Video Broadcasting-Asynchronous Serial Interface (DVB-ASI), which is a digital video signal standard, and the like. ESCON is IBM Corporation's registered trademark.
Transmission speeds of protocols of the above-described standards are different from one another. Transmission speeds of signals of a 1000BASE-SX and a 1000BASE-LX, which are general standards for Gigabit Ethernet, with a medium dependent interface (MDI) are 1250 Mbps. A transmission speed of Fibre Channel is 1062.5 Mbps. A transmission speed of ESCON is 200 Mbps. A transmission speed of DVB-ASI is 270 Mbps. None of these transmission speeds match with the multiplexing steps of the SONET/SDH Network.
One of the methods that have been made available to transmit a signal of a transmission speed unmatched with the multiplexing steps with a SONET/SDH frame uses a padding byte. According to this method, a frame having a payload band equal to/higher than the transmission speed of the signal to be transmitted is used. Then, in order to match the signal with the multiplexing steps of the SONET/SDH Network, the padding byte is inserted into an extra part of the payload.
In addition, a virtual concatenation standard has been drawn up in ITU-T G. 707 to improve transmission efficiency for the case when the signal having the transmission speed unmatched with the multiplexing steps is transmitted in a SONET/SDH frame.
In virtual concatenation, an optional number of paths are virtually integrated by using STS-1/VC-3 or STS-3c/VC-4 as a path unit to make a channel of a desired payload band.
For example, in order to receive a signal having a transmission speed of 1250 Mbps, nine STS-3c/VC-4's may be integrated to make a channel of a payload band set to 1347.84 Mb/s. According to the virtual concatenation standard, this channel is represented by STS-3c-9v/VC-4-9v.
Incidentally, 8B/10B block coding is employed for physical layers of the above-described Gigabit Ethernet, Fibre Channel, ESCON, and DVB-ASI. The 8B/10B block coding is described in detail in ANSI X3.230-1994, Fibre Channel Physical and Signaling Interface (FC-PH), pp.63-71.
In the 8B/10B block coding, data of an 8-bit unit is converted into a 10-bit code for every 8 bits in accordance with a predetermined coding rule. Original 8 bits are called a byte, and a 10-bit code that is a result of byte conversion is called a character. In the specification, they are respectively referred to as an 8B byte and a 10B character.
According to an 8B/10B coding rule, six or more identical codes never continue in a 10B-character signal. In addition, according to the 8B/10B coding rule, two opposite 10B characters of numerals “0” and “1” are set for each 8B byte. Then, one of the two 10B characters is selected in accordance with numerals “0” and “1” of former 10B character. Thus, because of many changing points contained in the 10B-character signal, a clock and data can be easily extracted.
The 10B character of the 8B/10B block code is defined to express data codes of 256 types and control codes of 12 types. Normally, a data code is represented by Dxx. Y, and a control code is represented by Kxx. Y. Each data code corresponds to 256 8B bytes represented by 8 bits. A combination of 10 bits, which are not used as a data code, is allocated to the control code. The control code is used for transmitting control information regarding a character synchronization pattern, link disconnection or the like. The 8B/10B block coding enables data to be transmitted in transparency, and also a variety of control information to be transmitted.
However, when a padding byte is inserted into a payload in order to multiplex the signal of which transmission speed unmatches with the multiplexing steps, transmission efficiency of the SONET/SDH frame declines.
As an example, consideration is now given to a case of transmitting a 1250 Mbps signal of Gigabit Ethernet. Since a payload band of STS-12c/VC-4-4c is 149.76 Mbps×4=599.04 Mbps, the signal of the transmission speed 1250 Mbps cannot be directly received. Thus, STS-48c/VC-4-16c higher than the STS-12c/VC-4-4 must be used. A payload band of the STS-48c/VC-4-16c is 2396.16 Mbps and, when the signal of the transmission speed 1250 Mbps is received, only 52% of the payload band is used. Therefore, transmission efficiency is extremely low.
On the other hand, transmission efficiency is improved by using the virtual concatenation when the signal of the transmission speed unmatched with the multiplexing steps is multiplexed in the SONET/SDH frame.
However, a signal is transmitted by a byte unit in the payload of the SONET/SDH. When a 10B character is transmitted through the SONET/SDH, a function to establish character synchronization by a control code is necessary. Consequently, the process is redundant.
Data of the 10B character is processed by a character unit. Thus, a unit of data processing is different from that of the SONET/SDH to complicate a processing circuit.
On the other hand, the 10B character may be decoded to 8B byte data to be transmitted. In transmission of an 8B-byte signal, the function to establish character synchronization is not necessary. As the 8B-byte signal is processed by a byte unit similar to that of the SONET/SDH, a processing circuit is simple. Moreover, a payload band necessary for transmitting 8B-byte data is 80% of that necessary for transmitting the 10B character. Therefore, it can be said that an 8B byte is suitable for transmitting data, reliability of which is supported by a high-order protocol.
For example, a transmission speed of the 10B character through Gigabit Ethernet is 1250 Mbps. When it is converted into an 8B-byte data, however, the transmission speed becomes 1000 Mbps. Accordingly, when the data of Gigabit Ethernet is decoded to 8B-byte data, and the virtual concatenation is applied, the data can be transmitted by STS-3c07v/VC-4-7v of a payload band of 1048.32 Mbps. That is, if seven STS-3c/VC-4's are connected, data of gigabyte Ethernet can be transmitted. Therefore, OC 48/STM-16 of a transmission speed 2488.32 Mbps enables data of 2-channel Gigabit Ethernet to be transmitted, whereby transmission efficiency can be greatly improved.
However, in a protocol using 8B/10B block coding, a frame boundary is identified by a control code. Accordingly, in practice, in order to decode the 10B-character data to 8B-byte data to transmit it in transparency, encapsulation must be carried out by another variable length frame technology.
Furthermore, in Fibre Channel and ESCON, boundary information (frame start information, end information or the like) or link state information of a data frame is transferred based on a plurality of control codes or a combination of a control code with a data code. Thus, if the data is simply decoded to 8B-byte data, control information transferred by a 10B character will be lost.